Outdoor materials such as wall protecting materials, roofing materials, solar battery panel materials, window materials, outdoor flooring materials, illumination protection materials, automobile members, and signboards comprise, as a constituent material, a laminate obtained by laminating a plurality of films to each other using an adhesive. Examples of the film composing the laminate include metal foils made of metals such as aluminum, copper, and steel; metal plates and metal deposited films; and films made of plastics such as polypropylene, polyvinyl chloride, polyester, fluororesin, and acrylic resin.
As shown in FIG. 1, a laminated sheet 10 is a laminate of a plurality of films 11 and 12, and the films 11 and 12 are laminated by interposing an adhesive 13 therebetween.
Since the laminate is exposed outdoors over a long term, excellent durability is required of the adhesive for laminated sheets. It is required for adhesives for laminated sheets, particularly adhesives for solar battery applications, which convert sunlight into electricity, to have a higher level of durability than a conventional adhesive for laminated sheets.
As shown in FIG. 3, in the case of solar battery applications, the laminated sheet 10 referred to as a back sheet is included in a solar battery module 1, together with a sealing material 20, a solar battery cell 30, and a glass plate 40.
Since the solar battery module 1 is exposed outdoors over a long term, sufficient durability against sunlight is required under conditions of high temperature and high humidity. Particularly, when the adhesive 13 has poor performance, the film 11 can become peeled from the film 12, and thus the appearance of the sheet 10 deteriorates. Therefore, it is required that the adhesive for laminated sheets for the production of the solar battery module does not undergo peeling of the film even if the adhesive is exposed to high temperature over a long term.
Patent Documents 1 to 3 disclose, as examples of adhesives for laminated sheets, urethane based adhesives for producing solar battery protection sheets.
Patent Document 1 discloses that a urethane adhesive for laminated sheets synthesized from an acrylic polyol is suited as an adhesive for solar battery back sheets (see Claim 1 and the paragraph number 0048).
Patent Document 2 discloses a protective sheet for solar battery modules in which an acrylic urethane resin is formed on a base material sheet (see Patent Document 2, Claim 1, and FIGS. 1 to 3).
Patent Document 3 describes mixing an isocyanate curing agent with an acrylic polyol to produce adhesives (see Table 1, Table 2); a solar battery back sheet is produced by using these adhesives (see paragraph 0107).
Patent Documents 1 to 3 teach that poor appearance of a solar battery module can be prevented by producing a solar battery back sheet using an adhesive which is excellent in hydrolysis resistance and laminate strength. However, it is hard to say that the adhesive sufficiently meets the high requirements of consumers. Furthermore, durability required of an adhesive for solar battery back sheets is getting higher year by year, and it is required for an adhesive for back sheets to have high adhesion. Since the solar battery module is mainly used outdoors, high adhesion at high temperature is required.
It is necessary that an adhesive for solar battery back sheets has sufficient adhesion even at high temperature and can maintain adhesion even when exposed outdoors over a long time, and also has an acceptable curing rate and has higher adhesion (particularly adhesion after aging) to a film base material. When a solar battery back sheet is produced by using the adhesives of Patent Documents 1 to 3, plural films composing the back sheet (laminated sheet) may be peeled under severe outdoor environments (at high temperature over a long period).
When heat between the back sheet 10 and the sealing agent 20 (see FIG. 3) excessively increases, the film 11 of the back sheet 10 and the sealing agent 20 are integrated with each other to form a relatively thick laminate (the sealing agent 20, the film 11 and the adhesive 13); the film 11 and the adhesive 13 (or the film 12) may sometimes cause interfacial peeling due to the influence of film thickening or heating history.
Patent Document 1: JP 2011-105819 A
Patent Document 2: JP 2010-238815 A
Patent Document 3: JP 2010-263193 A